The present invention pertains to apparatus for heat treating metal strip by convection heat transfer, particularly for curing and drying paint film applied to one or both sides of the strip.
In the coil coating industry, it has long been the practice to pass the painted strip through elongated horizontal ovens, in which the strip hangs in a relatively flat catenary curve from rollers set at the entrance and exit ends of the oven. Since the ovens are from approximatey 50 to 100 meters or more long, it is necessary to maintain considerable tension on the strip in order to keep the total sag within acceptable limits. With wide strip and in heavier gauges, the tension becomes quite large, and requires extremely heavy machinery and powerful drive motors to handle the load, which has the effect of making the machinery extremely expensive. Another serious problem is that in some cases the specific tension on the strip may become critical and/or excessive.
To avoid some of the difficulties and objections inherent in the above-described system, another form of strip transport has been devised, using jets of air impinging against the bottom and top surfaces of the strip to "float" the strip through the oven. This type of arrangement has many advantages, but there are times when the high velocity jets of air are detrimental, as when a wet paint film of low viscosity is being cured, in which case the jets of air may cause the paint to flow before it has taken its initial set.
In a production paint line which may coat strip of various gauges with many different kinds of paint having different characteristics, it will be found that there are some situations where the suspension oven with the strip hanging in a catenary curve is the best choice, whereas in other situations the flotation oven in which the strip is supported wholly or in part by air jets is the better choice. In still other situations, it may be desirable to have the strip hang in a catenary curve for a portion of the length of its transit through the oven, and then to be floated on air jets for another portion of the length of travel. With prior paint line ovens, this is not possible, as the oven is either a catenary curve oven or a floatation oven.